1. Field of the Invention
The present invention relates to an inductor device and a process of production thereof.
2. Description of the Related Art
The market is constantly demanding that electronic equipment be made smaller in size. Greater compactness is therefore required in the devices used in electronic equipment as well. Electronic devices originally having lead wires have evolved into so-called xe2x80x9cchip devicesxe2x80x9d without lead wires along with the advances made in surface mounting technology. Capacitors, inductors, and other devices comprised mainly of ceramics are produced using the sheet process based on thick film forming techniques or using screen printing techniques etc. and using cofiring process of the ceramics and metal. This enables realization of a monolithic structure provided with internal conductors and a further reduction of size.
The following process of production has been adopted to produce such a chip-shaped inductor device.
First, a ceramic powder is mixed with a solution containing a binder or organic solvent etc. This mixture is cast on a polyethylene terephthalate (PET) film using a doctor blade method etc. to obtain a green sheet of several tens of microns or several hundreds of microns in thickness. Next, this green sheet is machined or processed by laser etc. to form through holes for connecting coil pattern units of different layers. The thus obtained green sheet is coated with a silver or a silver-palladium conductor paste by screen printing to form conductive coil pattern units corresponding to the internal conductors. At this stage, the through holes are also filled with the paste for the electrical connection between layers.
A predetermined number of these green sheets are then stacked and press-bonded at a suitable temperature and pressure, then cut into portions corresponding to individual chips which are then processed to remove the binder and sintered. The sintered chips are barrel polished, then coated with silver paste for forming the terminations and then again heat treated. These are then electrolytically plated to form a tin or other coating. As a result of the above steps, a coil structure is realized inside of the insulator comprised of ceramics and thereby an inductor device is fabricated.
There have been even further demands for miniaturization of such inductor devices. The main chip sizes have shifted from the 3216 (3.2xc3x971.6xc3x970.9 mm) shape to 2012 (2.0xc3x971.2xc3x970.9 mm), 1608 (1.6xc3x970.8xc3x970.8 mm), and even further smaller shapes. Recently, chip sizes of 1005 (1xc3x970.5xc3x970.5 mm) have been realized. This trend toward miniaturization has gradually made the requirements for dimensional accuracy (clearance) on the steps severer in order to obtain stable and high quality.
For example, in an inductor device of a chip size of 1005, the stack deviation of the internal conductor layers is not allowed to exceed more than 30 xcexcm. If this is exceeded, remarkable variations occur in the inductance or impedance. In extreme cases, the internal conductors are even exposed. An inductor array device of a chip size of 2010(2.0xc3x971.0xc3x970.5 mm) having four coils within the single device has the same problems as described above.
In the case of an inductor device of a relatively large chip size of the related art, this stack deviation was not serious enough to have a notable effect on the properties of the device, but with a chip size of about 1005 or 2010, stack deviations have a tremendous effect on the device properties.
In the inductor devices of a relatively large size of the related art, the coil pattern units of the internal conductors in the different layers were L-shaped or reverse L-shaped. The L-shaped pattern units and reverse L-shaped pattern units were alternately stacked and through holes were provided at the ends of these patterns to connect the patterns of the different layers. The starting ends and finishing ends of the coil formed in this way were connected to leadout patterns.
Experiments by the present inventors etc. have shown, however, that when making the coil pattern units of the internal conductors at different layers L-shaped and reverse L-shaped and simply making the coil pattern units smaller in order to obtain a 1005, 2010, or other small-sized inductor device, the stack deviation of the internal conductors remarkably progresses.
The reason why the stack deviation progresses in a small-sized inductor device is believed to be as follows: That is, to obtain a predetermined inductance or impedance despite reduction of the chip size, it is necessary to increase the number of turns of the coil. Therefore, it is necessary to make each of the ceramic layers thinner. Further, a low resistance is required in the internal conductors, so it is not allowed to make the conductors thinner by the same rate as the ceramic sheet. Therefore, a smaller chip size results in a remarkable non-flatness of a green sheet after printing.
As a result, when applying pressure to superposed green sheets to form them into a stack, the conductor portions, which are relatively hard compared with the green sheets themselves, interfere with each other and therefore cause remarkable stack deviation. In particular, in a printing pattern based on the L-shapes of the related art, the stacked green sheets were pushed at a slant 3-dimensionally through the internal conductorsxe2x80x94which only aggravated the stack deviation. This phenomenon became a major hurdle to be overcome for stabilization of the quality of the device along with the increased reduction of the chip size of the devices.
Various proposals have been made to solve this problem. For example, Japanese Unexamined Patent Publication (Kokai) No. 6-77074 discloses to press printed green sheets in advance in order to flatten them. Further, Japanese Unexamined Patent Publication (Kokai) No. 7-192954 discloses to give the ceramic sheets grooves identical with the conductor patterns in advance, print the conductor paste in the grooves, and thereby obtain a flat ceramic sheet containing conductors. Further, Japanese Unexamined Patent Publication (Kokai) No. 7-192955 discloses not to peel off the PET film from the ceramic sheet, but to repeatedly stack another ceramic sheet, press it, then peel off the film. This method uses the fact that PET film undergoes little deformation and as a result could be considered a means for preventing stack deviation. Further, Japanese Unexamined Patent Publication (Kokai) No. 6-20843 discloses to provide a plurality of through holes along the circumference of the printed conductors so as to disperse the pressure at the time of press-bonding.
Each of the methods disclosed in the above publications added further steps to the method of stacking the ceramic sheets of the related art or made major changes in it. Further, they were more complicated than the method of the related art and therefore disadvantageous from the viewpoint of productivity.
An object of the present invention is to provide a process for the production of an inductor device able to suppress stack deviation without complicating the production processxe2x80x94even if the device is made smallerxe2x80x94and an inductor device made by that process.
The present inventors engaged in intensive studies of a process for production of a small-sized inductor device able to suppress stack deviation without complicating the production process and an inductor device produced by the same and as a result discovered that it is possible to suppress the stack deviation by suitably determining the repeating pattern shape of coil pattern units formed between insulator layers of the device and thereby completed the present invention.
According to the present invention, there is provided a process for the production of an inductor device comprising the steps of: forming a green sheet to form an insulating layer; forming a plurality of conductive coil pattern units on the surface of the green sheet so that a plurality of unit sections each including a single coil pattern unit are arranged on the surface of the green sheet and each two coil pattern units adjoining in the substantially perpendicular direction to the longitudinal direction of the unit sections are arranged centro-symmetrically with respect to a center point of a boundary line of adjoining unit sections; stacking a plurality of green sheets formed with the plurality of coil pattern units arranged centro-symmetrically and connecting the upper and lower coil pattern units separated by the green sheets to form a coil shape; and sintering the stacked green sheets.
In order to produce large numbers of inductor devices on an industrial scale, generally a plurality of coil pattern units are formed on the surface of a green sheet by screen printing etc. In the related art, these coil pattern units were all formed in the same orientation and same shape in every unit section of a single green sheet. Coil pattern units have to be able to be connected in the stacking direction in order to form coils and further have to such as to enable the cross sectional area of the coil to be made as large as possible within the limited area of the unit section, so normally have linear patterns extending along the longitudinal direction of the unit sections. The linear patterns in the coil pattern units extend along the longitudinal direction of the unit sections and are superposed in the stacking direction through green sheets, so the stacked green sheets tend to easily shift in a direction substantially perpendicular to the longitudinal direction of the linear patterns (longitudinal direction of unit sections). This tendency becomes more remmarkable as the device is made smaller, that is, as the area of the unit sections is made smaller.
In the process of production of an inductor device according to the present invention, each two coil pattern units adjoining in a direction substantially perpendicular to the longitudinal direction of the unit sections are arranged centro-symmetrically with respect to a center point of a boundary line of adjoining unit sections. Therefore, even if linear patterns of coil pattern units formed in the individual unit sections start to shift in the direction perpendicular to the linear patterns due to being superposed in the stacking direction, the linear patterns of the coil pattern units positioned below the adjoining unit sections will interfere with the shifting. As a result, in the present invention, it is possible to effectively prevent stack deviation particularly in a direction substantially perpendicular to the longitudinal direction of the unit sections (longitudinal direction of linear patterns). Note that the stack deviation in the longitudinal direction of the unit sections is inherently small and does not become a problem.
In the process of production according to the present invention, when forming the plurality of coil pattern units on the surface of the green sheet, preferably each two coil pattern units adjoining in the longitudinal direction of the unit sections are arranged at the same positions inside the individual unit sections. Alternatively, each two coil pattern units adjoining in the longitudinal direction of the unit sections may be arranged centro-symmetrically with respect to a center point of a boundary line of adjoining unit sections.
In the process of production according to the present invention, preferably the coil pattern units are each comprised of two substantially parallel linear patterns and a curved pattern connecting first ends of the linear patterns. Further, the coil pattern units are each comprised of line symmetric patterns about a center line dividing a unit section across its width direction. By making such coil pattern units, it is possible to further reduce the stack deviation while obtaining the desired inductor characteristics.
Further, preferably the plurality of green sheets are stacked so that each two coil pattern units adjoining each other in the stacking direction through a green sheet become line symmetrical with respect to a center line dividing the unit sections across the longitudinal direction. By stacking the green sheets in accordance with this positional relationship, it is possible to further reduce the stack deviation.
Further, preferably coil pattern units of a thickness of ⅓ to xc2xd of the thickness of the green sheets are formed on the surface of green sheets of a thickness of 3 to 25 xcexcm. When stacking relatively thin green sheets in this way, stack deviation easily occurs, but in the present invention it is possible to reduce the stack deviation even in such a case. Note that when the thickness of the coil pattern units exceeds ⅔ of the thickness of the green sheets, there is a tendency for suppression of the stack deviation to become difficult even in the present invention. When the thickness of the coil pattern units is smaller than ⅓ the thickness of the green sheets, there is little chance of the stack deviation becoming a problem, but the electrical resistance of the coil pattern units becomes largexe2x80x94which is not desirable for an inductor device.
Further, the process of production according to the present invention may include, before the sintering step, a step of cutting the stacked green sheets for each unit section or may include a step of cutting the stacked green sheets for each plurality of unit sections. By cutting the stacked green sheets for each unit section, it is possible to obtain an inductor device having a single coil inside the device. Further, by cutting the stacked green sheets for each plurality of unit sections, it is possible to obtain an inductor device having a plurality of coils inside the device (also called an xe2x80x9cinductor array devicexe2x80x9d).
According to the present invention, there is provided an inductor device comprising a device body having a plurality of insulating layers; a plurality of conductive coil pattern units formed inside the device body between insulating layers along a single planar direction, coil pattern units adjoining each other in the single plane being centro-symmetric patterns with respect to a center point of a boundary line between unit sections containing coil pattern units; and connection portions connecting upper and lower coil pattern units separated by the insulating layers to form a coil.
According to the present invention, it is possible to produce an inductor device by the above process of production of the present invention and possible to suppress stack deviation without complicating the production process even if the device is made small in size.